As a form of a press which efficiently performs a plurality of workings such as drawing, bending, piercing and trimming, on a single work, a tandem press line is known. In the tandem press line, a plurality of press apparatuses (hereafter referred to as “presses”) which are arranged in line are provided, and a work is transported from the press on the upstream side to that on the downstream side, being sequentially press formed by the respective presses. The work is transported by hand, a work transportation device, or the like. Each press is provided with a main motor, and the rotation of the main motor is converted into a vertical operation (a reciprocating motion) through a drive mechanism. The respective main motors are separately controlled, and thus the slides of the presses are vertically moved independently and individually. Hereinbelow, the press and the work transportation device will be described, respectively.
About Press:
FIG. 11 is a schematic diagram of the drive mechanism, in which, as an example of it, the drive mechanism of a crank press is shown. The rotational shaft of the main motor is provided with a motor pulley 52, and the motor pulley 52 is connected to a flywheel 54 through a belt 53. The flywheel 54 and a first engaging member of a clutch 55 are connected to each other, and a second engaging member of the clutch 55 is connected to a drive shaft 56. Further, the drive shaft 56 is provided with a brake 57. A part of the drive shaft 56 is engaged with a main gear 58, and the main gear 58 is fixedly installed to a part of a crankshaft 59. To the crank portion of the crankshaft 59, a slide 16 is suspended through connecting rods 45.
With this drive mechanism, the flywheel 54 is turned by the main motor 51, and the rotational energy of the flywheel 54 is transmitted to the crankshaft 59 through the clutch 55 and the main gear 58. Then, the crankshaft 59 is turned, and the rotation of it is converted into the vertical movement of the slide 16. By engaging or releasing the clutch 55, and releasing and applying the brake 57, the slide 16 is moved and stopped, respectively. The drive mechanism may be further provided with a transmission which contains a combination of a plurality of gear stages.
The slide vertical moving operation in each press is controlled as follows. When the slide 16 reaches the upper dead point, the clutch 55 is released with the brake 57 being applied, which stops the slide 16 at the upper dead point. When the work after the working is carried-out from the working station in the press, and further the work before the working is carried-in into the working station in the press, the clutch 55 is engaged, the brake 57 being released, which causes the slide 16 to be lowered from the upper dead point. Then, when the slide 16 passes through the lower dead point to reach the upper dead point, the clutch 55 is released and the brake is applied, which results in the slide 16 being again stopped at the upper dead point. Thus, with each press, engagement and release of the clutch 55, and release and application of the brake 57 are repeated, resulting in intermittent operation.
With a tandem press line, presses can be combined and arranged to suit to the application. In addition, when there is no need for using of a part of the presses in the tandem press line, the part of the presses can be stopped, or used for press forming of another work. Thus, the tandem press line can be applied for a variety of styles of press forming, and thus can have a high degree of freedom.
The transfer press, which is similar to the tandem press line, also has a plurality of working stations which are disposed in line. But with the transfer press, a single slide takes charge of a plurality of working stations, and a single main motor vertically moves the respective slides. Therefore, the slide vertical movements of the respective presses are synchronized. Consequently, the production efficiency is high, but the style of press forming is fixed, thus the degree of freedom is low.
About Work Transportation Device:
As the method for work transportation between presses which are adjacent to each other, the robot method and the loader and unloader method are known. The robot method refers to a method in which a multiarticulated type handling robot is disposed between presses which are adjacent to each other, and this handling robot carries out the work from the press for the previous process, and carries in that work into the press for the subsequent process. On the contrary to this, the loader and unloader method refers to a method in which a loader and an unloader having a link construction are provided by the side of the press body on the upstream side and the downstream side, respectively, and between the unloader on the upstream side and the loader on the downstream side, a shuttle carriage is provided, such that the carrying-out and carrying-in of the work from/into the press body are performed by the unloader and the loader, respectively, and the transporting of the work to the subsequent process is performed by the shuttle carriage.
However, with these conventional methods, the respective intermittent motions of the presses on the upstream side and the downstream side must be followed by the transporting of the work, and yet there is the need for preventing any interference from occurring between the work and the die or the like during transporting the work, which leads to problems in that the work handling cannot be performed at high speed, and thus the improvement of the production speed is limited. Further, with the robot method, teaching of the transporting locus is difficult and takes a long time. Also, with the loader and unloader method, a shuttle carriage must be provided between adjacent presses, which requires a large-scale apparatus to be constructed and a large space for the large-scale apparatus.
To solve these problems, the applicant of the present application has already proposed, as a prior Japanese application, a work transporting method and a work transportation device for tandem press line that allow teaching of the work transporting locus to be performed in a short time period, and yet allow the work transportation to be performed at high speed (Japanese Patent Application No. 2001-400849). The work transportation device of this prior application comprises a configuration that lifting beams which are in parallel with the work transporting direction and can be freely moved vertically are provided; carriers and sub-carriers which can be freely moved along the longitudinal direction of the lifting beams; and a cross bar having work holding means is provided between a pair of right and left sub-carriers.
The most efficient means for improving the production efficiency for a tandem press line is to continuously operate the respective presses, and further to cause the work transportation device to follow the press operation. However, in order to carry out such an operation, the following problems must be overcome.
For example, assuming that the respective slides of a plurality of presses are simultaneously lowered at the same speed from the upper dead point for press forming, if the respective slides perform the one stroke operation and return to the upper dead point with the same timing, no problems occur, but, actually the timings have a mutual discrepancy. This is because the respective presses are different in slowdown, and thus the period of the press operation differs from press to press.
The slowdown refers to a phenomenon in which the number of revolutions of the flywheel is temporarily lowered, and this phenomenon cannot be avoided due to the loading in the press forming, and the like. The slowdown depends upon a variety of factors, such as the energy required for a particular press forming, the capacity of the main motor, the size of the flywheel, and the like, however, these factors vary from press to press, thus the slowdown for the respective presses varies.
FIG. 12 is a diagram illustrating the slide position versus the elapsed time, showing the change in the slide position of adjacent presses when they are continuously operated. As shown with the waveforms A and B, even if a prescribed phase difference T1 is set for the slide operations of the adjacent presses, and the operation is started, the phase difference is gradually changed with the time elapse under the influence of the above-mentioned slowdown, resulting in the waveforms being changed into A and B′, for example. At the beginning, the amount of change in the phase difference is small, thus the carrying-out of the work from the press on the upstream side and the carrying-in of the work into the press on the downstream side can be continuously performed. However, as the time elapses, the amount of change in the phase difference is increased. Then when the amount of such change is increased to a certain degree, the timing relationship between the carrying-out of the work from the press on the upstream side and the carrying-in of the work into the press on the downstream side gets out of order so greatly that the operation of the line cannot be further continued.
With such a problem being presented, the conventional tandem press line has had to be intermittently operated in order to safely and positively carry-in and carry-out the work. Thus, the improvement of the production efficiency could not have been expected.
In addition, intermittent operation requires engagement and release of the clutch, and application of the brake. The engagement and release of the clutch, and the application of the brake involve high noise, and also cause the facing provided for the clutch and the brake to be worn. If the facing wear is heavy, the life of the facing is shortened, resulting in the facing replacement being required. Therefore, the maintenance cost is increased.
In addition, the invention of the prior application (Japanese Patent Application No. 2001-400849) is that which was proposed only about the hardware configuration of the work transportation device in order to provide high-speed work transportation in the tandem press line, thus there has been room for examination concerning the control technology for efficiently transporting the work while following up the respective presses in the intermediate transportation between two presses which are independently operated.
The present invention has been made in view of the above circumstances, and to achieve the purposes of improving the production efficiency of press forming and reducing the maintenance cost and the maintenance frequency.